Conventionally, a spark plug for ignition is used in an internal combustion engine such as an automobile engine. A spark plug in general has a structure in which an insulator with a center electrode insertedly provided therein is held by a metal shell in such a manner as to surround the periphery of the insulator, and a spark discharge gap is formed between the center electrode and a ground electrode joined to a leading end of the metal shell. The ignition of an air-fuel mixture flowing in between the both electrodes is effected by a spark discharge which is generated between the center electrode and the ground electrode.
When such a spark plug is used, a load accompanying the spark discharge, which is repeatedly effected in a combustion chamber which is set to high temperature in the neighborhood of 10,000° C., is applied to the electrodes, so that compatibility of spark wear resistance and high-temperature oxidation resistance is required for the electrode material used for the electrode. When the electrode material is affected by the load due to the high temperature and the spark discharge, crystal grains constituting the electrode material coarsen (undergo so-called grain growth), and the structure of their grain boundaries becomes simplified. Then, the ingress of oxygen into the interior of the electrode material becomes facilitated just as if the simplified intergranular structure forms guide passageways for oxygen, with the result that oxidative corrosion possibly becomes likely to occur in the interior.
Accordingly, to suppress the grain growth, an electrode material is known in which a metal element such as Y or Zr is added to Ni (e.g., refer to JP-A-2004-247175). In JP-A-2004-247175, an electrode material is formed in which a powder consisting of such as oxides or nitrides of these elements is mixed with an Ni powder, which mixture is quench-hardened after molding, allowing such as oxides or nitrides of the aforementioned elements to precipitate in the parent phase of Ni in a uniformly distributed state. In the electrode fabricated from such an electrode material, even if the electrode is affected by the load due to high temperature and spark discharge, such as oxides or nitrides precipitated in the parent phase of Ni suppresses in a pinning manner the coarsening of their crystal grains in the course of coarsening of the crystal grains, so that it is possible to suppress the grain growth. As the grain growth is suppressed, the grain size of the crystal grains is maintained in a small state. Since the structure of the grain boundaries is maintained in a relatively complex state because of it, the ingress of oxygen into the interior of the electrode along the grain boundaries is suppressed, so that the high-temperature oxidation resistance improves.
On the other hand, if the amount of the aforementioned elements added increases, it leads to an increase in the specific resistance of the electrode material and a decline in thermal conductivity, with the result that the spark wear resistance declines. In JP-A-2004-247175, by increasing the purity of Ni in the electrode material, the specific resistance of the electrode material is lowered and the thermal conductivity is improved, thereby enhancing the spark wear resistance.